Generally, a polymer membrane is very fragile due to its poor mechanical strength and is easily broken down when scratched by contaminants. Once contaminated by contaminants, the polymer membrane is hard to restore its original properties by physical cleaning (reverse-cleaning) and is very likely to be destroyed. Also, the polymer membrane has a relatively low chemical stability and thus when it is subjected to a chemical wash, it will result in decreasing its physical properties after repetitive washes. Further, the polymer membrane cannot be used to treat wastewater containing pyrogenic contaminants since it is not thermo-resistant.
In order to remedy the above-mentioned disadvantage of the polymer membrane, there has been introduced a ceramic membrane which has improved physical property such as mechanical strength, intensity of reverse-cleaning, intensity of chemical cleaning and thermo-resistance than polymer membrane has been introduced. However, the ceramic membrane is more limited in terms of its application than the polymer membrane because it is brittle.
Nowadays, a metallic membrane is manufactured industrially from GKN Company, Germany. It is prepared rather by a process comprising: (1) compressing metal particles in a frame to mold; and (2) sintering, not by the radiation process. Unfortunately, the metallic membrane is very expensive for the production and is not competitive in cost-wise to be used for water treatment. It is very small in the packing density per unit volume since tube-shaped in a large radius, compared with a hollow polymer membrane. Therefore, the metallic membrane is applied to only limited fields to be substituted for the polymer membrane.
Meanwhile, the hollow membrane is manufactured in a thread shape having a macaroni-like cavity and often utilized as a dialysis membrane to remove minute impurities.
Generally, there have been known a few methods to prepare the hollow membranes: i.e., (a) a method for preparing a flat sheet membrane, wherein inorganic powder is packed and pressed to prepare a membrane in a planar form; (b) a method for hydrolyzing metal alkoxides by a sol-gel process; (c) a method for exploiting a phase transition occurring between a solvent and a non-solvent; and (d) a radiation process for intruding and extruding a filaments-forming material in a plastic fusion or in a solution through an opening for radiation or an aperture in the frame with a certain rate.
The radiation process is broadly classified into three different kinds such as melt radiation, dry radiation and humid radiation, depending upon its modes. In detail, the melt radiation comprises: (1) dissolving polymers; (2) extruding into a liquid solution; and (3) solidifying in the air, under a gaseous atmosphere or in a freezer. The dry radiation is comprises: (1) extruding a polymer solution into a heated air to remove a solvent; and (2) solidifying it to prepare fibers. Finally, the humid radiation comprises: (1) extruding a polymer solution into a coagulation medium; (2) recycling the polymer; and (3) solidifying to prepare filaments.